Coal liquefaction is of major significance as an alternative synthetic fuel source. The conversion of coal into liquid (as well as gaseous) hydrocarbons according to existing technology can be carried out either by direct hydrogenation or through prior conversion to synthesis gas followed by Fisher-Tropsch synthesis. The existing processes are based upon technology developed in Germany during the 1920's employing improved engineering techniques.
Hydrogenation of coals producing liquefied products generally follows two main courses: solvent assisted hydrogenation at 300.degree. to 400.degree. C. and at 1000-4000 psi or higher temperature flash pyrolysis (600.degree. to 1000.degree. C.), either at ambient hydrogen pressure or hydrogen pressure up to 1500 psi. The solvent assisted liquefaction has the virtue of being able to obtain a high yield coal conversion to liquid products of relatively low molecular weight.
The use of catalysts in coal liquefaction processes causes, in general, significant difficulties. Coal is a solid material with very limited solubility in most common solvents (organic or inorganic). Thus, a major difficulty or problem in transforming coal catalytically is finding a means to bring hydrogen gas in proper contact with the coal. This fact obviously causes significant and as yet unresolved problems, if a solid catalyst is used. Even when employing a very fine mesh coal (mesh size 100.mu.), there is little surface contact. Also, the organic moiety of coals is a cross-linked polymeric material, which can only be partially dissolved or swelled by organic solvents. Thus, a homogeneous catalyst should also be preferentially soluble and compatible with solvents used or the reaction conditions should be such to allow the catalyst to make molecular contact with the large organic cross-linked molecules of coals. Further, the large polyaromatic polynuclear coal backbone must be depolymerized during the process to allow the formation of hydrogenated lower molecular weight hydrocarbons.
The phenol complex of boron trifluoride, a well defined acidic system (see G. A. Olah "Friedel-Crafts Chemistry", Wiley-Interscience New York, 1973, pp 247-248), was applied by Heredy in studies of depolymerization of coals and model compounds. (L. Heredy et al., Fuel, 41, 221 (1962), 42, 182 (1963), 43, 414 (1964), 44, 125 (1965)). This system is, however, a relatively weak acid system, which when heated slowly releases boron trifluoride starting at about 50.degree. C. Practically no boron trifluoride remains at the boiling point of phenol. No liquefaction of coal was reported in the phenol-boron trifluoride system, nor is it expected to be achieved due to the low acidity of the system and its inability to promote ionic hydrogenation. This system is thus well-recognized to be entirely different from the hydrogen fluoride-boron trifluoride superacid system of this invention (see Olah "Friedel-Crafts Chemistry" p. 244), a system previously used in the petrochemical industry, for example, for the isomerization of xylenes, but not applied previously in coal chemistry.
Friedel-Crafts type systems, such as zinc chloride or aluminum chloride-hydrochloric acid with hydrogen, were utilized previously in coal liquefaction, their use is of limited value because these acid catalyst systems cannot be readily regenerated, and in the latter case, results primarily in the formation of gaseous products, such as methane and ethane. Further, elevated reaction temperatures are needed in these energy consuming processes.
The application of Lewis acid catalyzed coal conversion has gained interest in recent years for producing liquid and gaseous products at temperatures between 200.degree. and 500.degree. C., generally 350.degree. to 450.degree. C. Zinc chloride in particular is utilized in the CONOCO process. Further it is known that active Lewis acid catalysts can be effective gasification catalysts under hydrocracking conditions by themselves (such as discussed by W. Kawa, S. Friedman, L. V. Frank and R. W. Hiteshue, Amer. Chem. Soc. Division of Fuel Chemistry, Vol. 12, No. 3, 43 (1963)) or with Lewis acid protic acid conjugated superacid systems, such as aluminium chloride and hydrochloric acid (J. Y. Low and D. S. Ross, ibid, 22, No. 7, 118 (1977).
U.S. Pat. No. 4,202,757, to Amendola issued May 13, 1980, describes the rapid conversion of coal to a high percentage of liquid hydrocarbons by first reacting it with an acid to form carbon addition products, which are then reacted with a Group V halide ion acceptor system (i.e., superacid system), such as antimony pentahalides, and thereafter with a hydrogen donor source. All phases are claimed to be carried out at atmospheric pressure and relatively low temperatures (150.degree. to 500.degree. C.).
Zinc chloride and aluminum chloride as well as the related Lewis acid halides of high redox potentials, are described as applicable in these processes, but are extremely difficult to recover due to their limited volatility and strong complexing with the basic sites abundant in coal. The Group V halides claimed by U.S. Pat. No. 4,202,757 are generally unsuitable and impractical catalysts for coal conversion because their hydrolytic ability and generally high chemical reactivity result in irreversible reactions with coals. Also, their redox potential is low, and they are thus easily reduced under the reaction conditions. Antimony pentahalides, for example, generally are not compatible, as is well known to those familiar with superacid chemistry, with hydrogen or hydrogen donors. Further, antimony pentahalides are extremely reactive with water and any other nucleophiles abundant in coals or other carbonaceous materials. As known to those familiar with their chemistry, when reacted with coals, antimony pentafluoride or its conjugate superacids give insoluble, rock-like materials, which are neither converted to hydrocarbon oils or gases and do not allow recovery of the halide. Due to these difficulties and despite appreciable effort, none of the catalytic processes described in U.S. Pat. No. 4,202,757 has so far resulted in any practical process of improved nature.